1. Field of the Invention
The present invention relates generally to a semiconductor memory device, and more particularly, to a semiconductor memory device having integrated dynamic oxide semiconductor random access memory (hereinafter DOSRAM) and non-volatile oxide semiconductor random access memory (hereinafter NOSRAM). A method of making such semiconductor memory device is also disclosed.
2. Description of the Prior Art
Semiconductor memory is an electronic data storage device, often used as computer memory, implemented on a semiconductor-based integrated circuit. It is made in many different types and technologies. Semiconductor memory has the property of random access, which means that it takes the same amount of time to access any memory location, so data can be efficiently accessed in any random order.
One type of memory is dynamic random access memory (DRAM). DRAM is dynamically accessible. In DRAM, memory is stored for a short period of time, typically less than a second, and accordingly DRAM requires periodic refreshing. DRAM loses memory when power is off or fails, and accordingly is volatile. DRAM may be accessed and altered quickly.
Another type of memory is static random access memory (SRAM). SRAM may be accessible faster than DRAM, may store memory permanently as long as power is present, and thus may not require refreshing like DRAM. SRAM, like DRAM, loses memory when power is off, and accordingly is also volatile.
While DRAM traditionally utilizes unit cells containing a transistor and a capacitor, SRAM unit cells comprise six transistors, and therefore the cell size for SRAM is much larger than that for DRAM. In high-performance electronic systems, SRAM may be used for instruction storage, while DRAM is used for working memory and short-term data storage.
Still another type of memory is nonvolatile memory (NVM). NVM is memory that stores data without refreshing, and accordingly memory suitable for storing data for substantial lengths of time (for instance, 10 years) in the absence of power.
Both DRAM and NVM offer advantageous and disadvantages. DRAM is fast, but the volatility of DRAM leads to data loss once power is turned off, and the refreshing of DRAM consumes power. NVM is stable in the absence of power, but tends to be slower to program than DRAM. Also, NVM often utilizes higher programming voltages than DRAM, which lowers the endurance of NVM relative to DRAM.
It is desirable to provide a semiconductor memory device and a fabrication method thereof, which are able to integrate high-density DRAM and low-leakage NVM embedded in one chip.